The country's longest-running singing competition, "Tawag ng Tanghalan," opens its doors to students from Luzon, Visayas, and Mindanao as they represent their school and showcase their singing prowess in "Tawag in Tanghalan: School Showdown."Two contenders from different universities will go head-to-head and impress the hurados with their golden voices in the eight season of “It’s Showtime’s”…